Paypal requires a Third Party Permission Username (?), where do I get this?

I am installing Paypal on a new website (www.pickleballpaddleworld.com not live on new host) the install requires various credentials which I got from my Paypal account and entered on the install screen and the install was successful, I do get the Paypal button at checkout . But on my first transaction I get an error returned from Paypal of "(10002) 10002 Security error - Security header is not valid". When I contacted Paypay they said I need to provide them with a Third Party Permission Username. I have contacted my website host and they do not provide that information. Does anyone know where I would get a Third Party Permission Username for the Zencart Paypal Express Checkout? Some added info: www.pickleballpaddleworld.com is live on another host which I am moving away from in early June 2018.

Did you use the same account to log in to PayPal as the one used for Zen Cart? That will not work, as you can not make a payment to yourself.